Technical Field
The present invention relates to an optical system for an infrared ray used in an infrared wavelength range.
Background Art
Conventionally, optical systems for an infrared ray are used for analysis of heat distribution of an object in medical and industrial fields in addition to surveillance and personal authentication. In recent years, applications of optical systems with an infrared ray have spreading into onboard cameras, and are also spreading into various applications not only surveillance, travel record and driving support but also detection of pedestrians and obstacles at night. With increasing in the applications of the optical systems for an infrared ray, a fixed focal length lens having a small F-number which can provide a bright image has been demanded rather than a zoom lens with a variable focal length.
The optical system for an infrared ray is generally constituted by a combined plurality of lenses made of infrared transmitting materials including germanium having a high refractive index for an infrared ray. For example, Patent Document 1 discloses a wide-angle optical system for an infrared ray constituted by a first lens composed of a meniscus lens whose convex surface faces an object side, and a second lens and a third lens composed of positive meniscus lenses whose convex surfaces face the object side these are arranged sequentially from the object side. Patent Document 2 discloses a bright optical system for an infrared ray constituted by a first lens composed of a positive meniscus lens whose convex surface faces an object side, and a second lens composed of a positive meniscus lens whose convex surface faces an imaging side these are arranged sequentially from the object side and has a small F-number.